Self-aligned emitter-base in advanced BiCMOS technology

ABSTRACT

A self-aligned bipolar transistor and method of fabricating the same are disclosed. In an embodiment, a substrate and an intrinsic base are provided, followed by a first oxide layer, and an extrinsic base over the first oxide layer. A first opening is formed, exposing a portion of a surface of the extrinsic base. Sidewall spacers are formed in the first opening, and a self-aligned oxide mask is selectively formed on the exposed surface of the extrinsic base. The spacers are removed, and using the self-aligned oxide mask, the exposed extrinsic base and the first oxide layer are etched to expose the intrinsic base layer, forming a first and a second slot. A silicon layer stripe is selectively grown on the exposed intrinsic and/or extrinsic base layers in each of the first and second slots, substantially filling the respective slot.

BACKGROUND OF THE INVENTION

The invention relates generally to self-aligned bipolar transistors.More particularly, the invention relates to self-aligned emitter-basetransistors having a selectively grown silicon link between theintrinsic and extrinsic bases.

Bipolar transistors having, for example, a silicon germanium (SiGe)intrinsic base and a polysilicon extrinsic base are frequently used inintegrated circuits fabricated for high performanced mixed signalapplications. In bipolar transistors (BTs) and high performanceheterojunction bipolar transistors (HBTs), it is desirable to haverelatively small size, high cutoff frequency (F_(t)) and maximumoscillation frequency (F_(max)). F_(max) is a function of F_(t) and ofparasitics, including parasitic capacitances and parasitic resistances.Exemplary parasitics include but are not limited to: collector-basecapacitance C_(cb), base-emitter C_(be) capacitance, and base resistanceR_(b). It has been a challenge, however, to fabricate very smalltransistors that operate in, for example, the 90 nanometer (nm)technology node, which are capable of accurately amplifying electricalsignals at a frequency of about 300 GHz, and of being integrated with 90nm CMOS devices as well as other passives and other features.

BRIEF DESCRIPTION OF THE INVENTION

A first aspect of the disclosure provides a method of fabricating aself-aligned bipolar transistor. The method includes: providing asubstrate, and providing a stack of films disposed above the substrate,the stack of films including an intrinsic base over the substrate, afirst oxide layer over the intrinsic base, an extrinsic base over thefirst oxide layer, a second oxide layer over the extrinsic base, anitride layer over the second oxide layer, and a third oxide layer overthe nitride layer. A first opening is formed in the third oxide layer,the nitride layer, and the second oxide layer, exposing a surface of theextrinsic base. A spacer is formed on a sidewall of the first opening,and a self-aligned oxide mask is formed on the exposed surface of theextrinsic base. The spacer is then removed, and using the self-alignedoxide mask, the exposed extrinsic base and the first oxide layer areetched to expose the intrinsic base layer, forming a first slot and asecond slot. A silicon layer stripe is selectively grown in each of thefirst and second slots, wherein the silicon layer stripe in each of thefirst and second slots substantially fills the first and second slots.

A second aspect of the disclosure provides a method of fabricating aself-aligned bipolar transistor. The method comprises forming a firstopening to expose a surface of an extrinsic base region, and forming aspacer on a sidewall of the first opening. A self-aligned oxide mask isformed on the exposed surface of the extrinsic base, and the spacer isremoved. The self-aligned oxide mask is then used to form a self-alignedfirst slot and a self-aligned second slot extending vertically through athickness of the exposed extrinsic base and a first oxide layer toexpose an intrinsic base layer. A silicon layer stripe is thenselectively grown on at least one of the exposed intrinsic base layerand extrinsic base layer in each of the first slot and the second slot,wherein the silicon layer stripe in each of the first and second slotssubstantially fills the first and second slots.

A third aspect of the disclosure provides a self-aligned bipolartransistor comprising: an intrinsic base layer disposed above asubstrate; an oxide layer disposed above the intrinsic base layer; anextrinsic base layer disposed above the oxide layer; a first slot and asecond slot extending vertically through a thickness of each of theextrinsic base layer and the oxide layer; and a silicon layer stripedisposed in each of the first slot and the second slot, wherein thesilicon layer stripe is selectively grown on a surface of at least oneof the intrinsic base layer and the extrinsic base layer, wherein thesilicon layer stripe in each of the first and second slots substantiallyfills the first and second slots.

These and other aspects, advantages and salient features of theinvention will become apparent from the following detailed description,which, when taken in conjunction with the annexed drawings, where likeparts are designated by like reference characters throughout thedrawings, disclose embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-11 show cross-sectional views of a partially completed bipolartransistor formed in accordance with embodiments of the invention.

FIGS. 12-13 show cross sectional views of a partially completed bipolartransistor in accordance with embodiments of the invention.

FIG. 14 shows a cross sectional view of a partially completed bipolartransistor having a gap in a silicon layer stripe.

FIGS. 15-20 show cross-sectional views of a partially completed bipolartransistor in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the invention and the various features thereof areexplained more fully with reference to the embodiments depicted in thedrawings.

As discussed above, it is desirable in bipolar transistors (BTs), andparticularly in high performance heterojunction bipolar transistors(HBTs) to have a relatively high cutoff frequency (F_(t)) and maximumoscillation frequency (F_(max)). With reference to FIGS. 1-7,embodiments of the disclosure provide an improved transistor 100 and amethod for fabricating transistor 100.

Referring to FIG. 1, embodiments of transistor 100 (which may be a BT oran HBT structure) include a substrate 120 having a first conductivitytype (e.g., P-type conductivity). For example, substrate 120 may becrystalline silicon, or more specifically, P-silicon (i.e., siliconlightly doped with a P-type dopant). Substrate 120 may further comprisea collector region 122 having a second conductivity type, which isdifferent from the first conductivity type (e.g., N-type conductivity).Various configurations of BT and HBT collector regions 122 are known inthe art and may be incorporated into transistor 100. For example,collector region 122 may include a single N-well region, or multipleN-type collector components. Shallow trench isolation (STI) regions 123can optionally be positioned within and at the top surface 115 of thesemiconductor substrate 120 to define the active area of the device.Specifically, the STI regions 123 can comprise relatively shallowtrenches patterned and etched into the top surface of the substrate 120around (i.e., bordering) an area designated as the active area of thedevice. The trenches can be lined (optionally) and filled with one ormore isolation materials (e.g., a silicon oxide material, a siliconnitride material, a silicon oxynitride material or any other suitableisolation material or combination thereof).

As shown in FIG. 2, intrinsic base layer 108, having the firstconductivity type (e.g., P-type conductivity), is disposed on top ofsurface 115 of substrate 120, above and in electrical contact withcollector region 122. Intrinsic base layer 108 may further extendlaterally over STI regions 123. Intrinsic base layer 108 may comprisesilicon (Si) or silicon germanium (SiGe), and may be formed, e.g., by acontemporaneous epitaxy process or subsequent implantation. Intrinsicbase layer 108 may further be doped with a predetermined concentrationof P-type dopant.

As shown in FIG. 3, first oxide layer 124 may be deposited overintrinsic base layer 108. First oxide layer 124 may comprise, e.g., anoxide such as silicon oxide or silicon germanium oxide, as appropriateto the composition of intrinsic base layer 108. As shown in FIG. 4, araised extrinsic base layer 101, having the same conductivity type(e.g., P-type conductivity) as intrinsic base layer 108, may bedeposited above first oxide layer 124. First oxide layer 124 thusphysically separates intrinsic base layer 108 and extrinsic base layer101. Extrinsic base layer 101 may be silicon or polysilicon in variousembodiments. In one embodiment, extrinsic base layer 101 may be, e.g.,heavily P+ doped polysilicon, and intrinsic base layer 108 may be P+doped SiGe, forming part of an NPN transistor 100.

As shown in FIG. 5, at least one dielectric layer, and preferably astack 125 of dielectric layers is deposited over extrinsic base layer101 using conventional deposition processes. Stack 125 may includesecond oxide layer 126 (which may include silicon oxide) disposed overextrinsic base 101, nitride layer 130 (which may include siliconnitride) disposed over second oxide layer 126, and third oxide layer 128(which may include silicon oxide) disposed over nitride layer 130. Eachlayer is deposited at least over intrinsic base layer 108.

As shown in FIG. 6, a first opening 132 is formed in oxide-nitride-oxidestack 125 using an etch 134, exposing a portion of the upper surface ofextrinsic base 101, and forming vertical sidewalls 131. This may be doneusing a mask to pattern first opening 132 so that it is positioned abovecollector region 122, and further, so that it may be substantiallycentered between STI regions 123. An anisotropic etch, such as areactive ion etch (RIE) may be used to etch through the one or moredielectric layers in stack 125. As shown in FIG. 7, sacrificial spacers136 are formed on the sidewalls 131 of first opening 132, according toconventional methods. In an embodiment, sidewall spacers 136 may includea nitride such as silicon nitride. As shown in FIG. 8, after the spacers136 are formed, a self-aligned oxide mask 138 is formed on the exposedsurface of extrinsic base 101. In an embodiment, self-aligned oxide mask138 may be formed using HI Pressure OXidation (HIPOX) to oxidize apolysilicon extrinsic base 101, resulting in a silicon oxide mask 138grown on a polysilicon extrinsic base 101.

As shown in FIG. 9, spacers 136 may then be selectively removed using,e.g., a wet etch such as hot phosphoric acid, or reactive ion etching(RIE). After spacers 136 are removed, the exposed portions 140, 142 ofextrinsic base layer 101 are the only exposed silicon material on thedevice; oxide mask 138 and the remaining portions of stack 125 protectthe balance of extrinsic base layer 101.

As shown in FIG. 10, using self-aligned oxide mask 138, the exposedportions 140, 142 (FIG. 9) of extrinsic base layer 101 and theunderlying portions of first oxide layer 124 are etched 145 to expose aportion of a surface of intrinsic base layer 108 in first slot 144 and asecond slot 146. Etch 145 may be an anisotropic etch such as RIE. Thus,the resulting first and second slots 144, 146 substantially conform tothe perimeter of first opening 132, such that it is substantiallyannular with respect to first opening 132.

As shown in FIG. 11, silicon layer stripes 148, 150 may be selectivelygrown on the exposed portions of at least one of intrinsic base layer108 and extrinsic base layer 101 in each of the first and second slots144, 146 (labeled in FIG. 10) Silicon layer stripes 148, 150substantially fill each of first and second slots 144, 146 withoutoverburdening first and second slots 144, 146. In other words, slots144, 146 are not overfilled; rather, the entirety of silicon layerstripes 148, 150 can be contained within first and second slots 144, 146respectively. In various embodiments, silicon layer stripes 148, 150 mayinclude one of Si or SiGe. In further embodiments, the Si or SiGesilicon layer stripes 148, 150 may be doped in situ with boron or otherdopants as known in the art to further reduce base resistance (R_(b)).Silicon layer stripes 148, 150 provide a link placing intrinsic base 108and extrinsic base 101 in electrical signal communication.

As mentioned above, the step shown in FIG. 11 includes selectivelygrowing silicon layer stripes 148, 150 without overburdening first andsecond slots 144, 146, in contrast with the embodiment of FIGS. 12-13,which illustrates a non-selective deposition of silicon layer stripeswith over burden 153 (FIG. 12), followed by an isotropic etch 155 (FIG.13) to remove silicon overburden 153 above slots 144, 146. Referringback to FIG. 11, as noted, silicon layer stripes 148, 150 are insteadselectively grown on exposed surfaces of one or both of intrinsic baselayer 108 and extrinsic base layer 101. Accordingly, pressure, amongother conditions in the chamber, may be adjusted as known and practicedin the art to allow for control of selective growth of silicon layerstripes 148, 150 as shown in FIG. 11. For example, a rate of growthinward from the sides of first and second slots 144, 146 in contact withextrinsic base layer 101, and a rate of growth upward from intrinsicbase 108 must be controlled so as to form substantially solid siliconlayer stripes 148, 150 as shown in FIG. 11, and so as to avoid formingsilicon layer stripes 148, 150 that include a void 158 in a center offirst and/or second slot 144, 146 as shown in FIG. 14.

FIGS. 15-20 depict further processing steps for forming an emitter abovecollector region. As shown in FIGS. 15-17, after silicon layer stripes148, 150 are formed, conventional sidewall spacer formation techniquescan be used to form a first portion 161 of a dielectric spacer 160 onthe first vertical sidewall 131 of the first opening 132 so that it isabove silicon layer stripes 148, 150. For example, a nitride layer canbe deposited and an anisotropic etch process (e.g., a reactive ion etch(RIE) process) can be performed in order to shape the first portion 161of the dielectric spacer 160, covering the entirety of the top surfaceof silicon layer stripes 148, 150.

As shown in FIG. 16, a second opening 225 can be formed within the firstopening 132 by selectively removing exposed portions of the sacrificialdielectric layer 138 (FIG. 15) and the extrinsic base layer 101 notprotected by the first portion 161 of the dielectric spacer 160. Thiscan be accomplished by performing selective anisotropic etch processes(e.g., selective reactive ion etch (RIE) processes), stopping on thefirst oxide layer 124. As shown in FIG. 17, a second portion 162 of thedielectric spacer 160 can be formed in the second opening 225 on theexposed first oxide layer 124 and positioned laterally adjacent to thefirst portion 161 and the extrinsic base layer 101. This can beaccomplished, for example, by depositing a nitride layer and performingan anisotropic etch process (e.g., a reactive ion etch (RIE) process) inorder to shape the second portion 161 of the dielectric spacer 160.

As shown in FIG. 18, after dielectric spacer 160 is formed, a thirdopening 235 (i.e., an emitter layer opening) can be formed byselectively removing any exposed first oxide layer 124 not protected bythe second portion 161 of the dielectric spacer 160. This can beaccomplished by performing a selective anisotropic etch process (e.g., aselective reactive ion etch (RIE) process), stopping on the intrinsicbase layer 108.

As shown in FIG. 19, emitter layer 180, having a same conductivity typeas collector region 122 (e.g. N-type conductivity), can be formed on theexposed intrinsic base layer 108 within the emitter layer opening 235(FIG. 18) such that it is positioned laterally adjacent to the firstoxide layer 124 and dielectric spacer 160. Thus, the first oxide layer124 and dielectric spacer 160 electrically isolate the emitter layer 180from the extrinsic base layer 101 and silicon layer stripes 148, 150.This can be accomplished by depositing a semiconductor layer so as tofill the emitter layer opening 235 and the remaining space within thefirst and second openings 132, 225, as defined by the dielectric spacer160. The geometry of the emitter layer 180 is essentially defined by thedielectric spacer 160 because the second portion 161 of the dielectricspacer 160 functions as a mask for defining the emitter layer opening235 and, thereby defines the width (i.e., diameter) of the relativelynarrow lower portion 181 (FIG. 19) of emitter layer 180 in contact withthe intrinsic base layer 108.

After the emitter layer 180 is formed, additional processing can beperformed to complete the transistor 100. This additional processing canalso include, but is not limited to, silicide layer 190 formation (FIG.20), interlayer dielectric formation, contact formation (not shown),etc. For example, to form the silicide layers 190, a mask can be formedover the top surface 185 of the emitter layer 180 and the stack 125 ofone or more dielectric layers can be etched back to expose the topsurface 145 of the raised extrinsic base layer 101. Then, the mask canbe removed and a cobalt, platinum, nickel or other suitable conductingmetal silicide layer can be formed, using conventional metal silicideprocessing techniques, on the exposed surfaces of the extrinsic baselayer 101 and emitter layer 180.

In addition to the method provided in FIGS. 1-11, also provided is thetransistor 100 fabricated according to that method. Transistor 100 mayinclude intrinsic base layer 108 disposed above substrate 120. Substrate120 may include shallow trench isolations 123, with collector region 122there between. First oxide layer 124 may be disposed above intrinsicbase layer 108, and extrinsic base layer 101 may be disposed above firstoxide layer 124. Additional layers, such as second oxide layer 126,nitride layer 130, and third oxide layer 128 may be disposedsequentially above extrinsic base layer 101. A first slot 144 and asecond slot 146 (labeled in FIG. 10) may extend vertically through thefull thickness of each of extrinsic base layer 101 and first oxide layer124. Silicon layer stripes 148, 150 may be disposed in each of first andsecond slots 144, 146, and each substantially fill first and secondslots 144, 146 respectively with substantially no overgrowth above theslots. As discussed above, silicon layer stripes 148, 150 areselectively grown on a surface of intrinsic base layer 108, rather thandeposited as a blanket over the device, and may comprise one of Si orSiGe. The depth of silicon layer stripes 148, 150 can be controlled viaa variety of factors including pressure in the chamber during growth. Insome embodiments, the silicon layer stripes 148, 150 may be doped insitu with boron to further reduce signal resistance. Silicon layerstripes 148, 150 provide an electrical signal connection betweenintrinsic base layer 108 and extrinsic base layer 101, and thereforebetween a subsequently-formed emitter 180 (FIG. 20), base, and collectorregion (122) in transistor 100.

It should be understood that in the above-described embodiments, anycomponent formed with an N-type conductivity will be doped (e.g., eitherin-situ doped, subsequently implanted, etc.) with an N-type conductivitydopant, and any component formed with a P-type conductivity will bedoped (e.g., either in-situ doped, subsequently implanted, etc.) with aP-type conductivity dopant. Such N-type conductivity dopants cancomprise, for example, Group V dopants, such as arsenic (As),phosphorous (P) or antimony (Sb) and such P-type conductivity dopantscan comprise, for example, Group III dopants, such as boron (B) orindium (In)).

It should further be understood that the terminology used herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of the invention. For example, for illustrationpurposes, the transistor 100 was described in detail above with respectto an NPN transistor (i.e., a transistor in which the collector has anN-type conductivity, the base has a P-type conductivity and the emitterhas an N-type conductivity). Similarly, the method of the presentinvention was described in detail above with respect to forming an NPNtransistor. However, it should be understood that this description isnot intended to be limiting and that the novel structure and methodcould also apply to a PNP transistor (i.e., a transistor in which thecollector has a P-type conductivity, the base has an N-type conductivityand the emitter has a P-type conductivity).

As used herein, the terms “first,” “second,” and the like, do not denoteany order, quantity, or importance, but rather are used to distinguishone element from another, and the terms “a” and “an” herein do notdenote a limitation of quantity, but rather denote the presence of atleast one of the referenced item. The modifier “about” used inconnection with a quantity is inclusive of the stated value and has themeaning dictated by the context (e.g., includes the degree of errorassociated with measurement of the particular quantity). The suffix“(s)” as used herein is intended to include both the singular and theplural of the term that it modifies, thereby including one or more ofthat term (e.g., the metal(s) includes one or more metals). Rangesdisclosed herein are inclusive and independently combinable (e.g.,ranges of “up to about 25 mm, or, more specifically, about 5 mm to about20 mm,” is inclusive of the endpoints and all intermediate values of theranges of “about 5 mm to about 25 mm,” etc.).

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of fabricating a bipolar transistor, themethod comprising: providing a substrate; providing a stack of filmsdisposed above the substrate, the stack of films including an intrinsicbase over the substrate, a first oxide layer over the intrinsic base, anextrinsic base over the first oxide layer, a second oxide layer over theextrinsic base, a nitride layer over the second oxide layer, and a thirdoxide layer over the nitride layer; forming a first opening in the thirdoxide layer, the nitride layer, and the second oxide layer, exposing asurface of the extrinsic base; forming a spacer on a sidewall of thefirst opening; forming a self-aligned oxide mask on the exposed surfaceof the extrinsic base; removing the spacer; using the self-aligned oxidemask, etching the exposed extrinsic base and the first oxide layer toexpose the intrinsic base layer, forming a first slot and a second slot;and selectively growing a silicon layer stripe on at least one of theexposed intrinsic base layer and extrinsic base layer in each of thefirst slot and the second slot, wherein the silicon layer stripe in eachof the first and second slots substantially fills the first and secondslots.
 2. The method of claim 1, wherein the extrinsic base comprisespolysilicon.
 3. The method of claim 2, forming the self-aligned oxidemask on the exposed surface of the extrinsic base further includes usingHI Pressure OXidation (HIPDX) to oxidize the polysilicon extrinsic base.4. The method of claim 1, wherein the intrinsic base comprises silicongermanium (SiGe).
 5. The method of claim 1, wherein selectively growingthe silicon layer stripe in each of the first slot and the second slotfurther comprises selectively growing one of Si and SiGe.
 6. The methodof claim 5, wherein selectively growing the silicon layer stripe in eachof the first slot and the second slot further comprises in situ dopingwith boron.
 7. The method of claim 1, wherein selectively growing thesilicon layer stripe in each of the first slot and the second slotplaces the intrinsic base and the extrinsic base in electrical signalcommunication.
 8. The method of claim 1, wherein the spacer comprises anitride, and removing the spacers further comprises using one of areactive ion etch and a wet etch using hot phosphoric acid.
 9. Themethod of claim 1, wherein selectively growing the silicon layer stripeon the exposed intrinsic base layer in each of the first slot and thesecond slot further comprises adjusting a pressure in an environment.10. A method of fabricating a bipolar transistor, the method comprising:forming a first opening in at least one film disposed above an extrinsicbase region to expose a surface of the extrinsic base region; forming aspacer on a sidewall of the first opening; forming a self-aligned oxidemask on the exposed surface of the extrinsic base; removing the spacer;using the self-aligned oxide mask, forming a self-aligned first slot anda self-aligned second slot extending vertically through a thickness ofthe exposed extrinsic base and a first oxide layer disposed below theextrinsic base to expose an intrinsic base layer; and selectivelygrowing a silicon layer stripe on at least one of the exposed intrinsicbase layer and the exposed extrinsic base layer in each of the firstslot and the second slot, wherein the silicon layer stripe in each ofthe first and second slots substantially fills the first and secondslots.
 11. The method of claim 10, wherein the extrinsic base comprisespolysilicon, and the self-aligned oxide mask comprises silicon oxide.12. The method of claim 10, wherein the intrinsic base further comprisessilicon germanium (SiGe).
 13. The method of claim 10, wherein theselectively grown silicon layer stripe in each of the first slot and thesecond slot further comprises SiGe in situ doped with boron.